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Discussion Starter #1
I'd just got done replacing the U joint to the rear end and took the Cat out to test it out. Smooth - waaaay smooth. I was quite happy.

So last night I take it out the freeway for the first time since I've redone the steering, suspension, brakes, etc etc etc. I'd also changed out the rear in favor of a 9" 3.00 Granada w/ factory discs which I pulled from my parts car. It stops on a dime now but that's beside the point. It drives fine til probably around 60 mph and up. From there on there's a steady humming vibration coming from somewhere around the driveline or rear end itself. Can't be sure. It's definitely not as bad as a failed U joint - but it's annoying enough to make me not want to go very far at higher speeds.

I realize this description might be a tad ambiguous, but does that sound like it might be a slightly imbalanced driveline? Maybe worn gears in the rear or just short of lube back there?
 

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might be the shaft balance is off. Unbolt and rotate 180 deg. at rear end. Then take it for a test drive... Could also be improperly set up ring and pinion, but if it is a stock rear end, prolly not likely.
 

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Sounds like a bent or imbalanced driveshaft.

Checking for bend is easy. with the rear end up on jackstands and the car in Neutral, have someone rotate the rear tires while you're under there with another jackstand (for a tool rest) and a screwdriver or marker. I think you get the idea -- pretend the driveshaft is something on a lathe and lightly touch the tool to the shaft. Do this at the front, center and rear of the shaft.

My old 85 T-bird Turbo Coupe developed a driveline vibration. It turns out that I must have kicked up a piece of road debris and it hit the middle of the driveshaft (either that or I high-centered on a rock, but I don't off-road ;) ) with enough force to put a spiral dent in it about a foot long and a quarter inch deep at the deepest point. Using the method described above I was able to determine that the shaft got bent. Time for a new shaft from the junkyard...

However, it could also be a slightly imbalanced tire or bent axle. The tire imbalance should be easy to spot, but a bent axle is both extremely rare and tough to diagnose.
 

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When you did your u-joint did you pull the driveshaft apart?
You have to put it together exactly the same way or it will be out of balance.
 

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Discussion Starter #5
Well, if pull apart you mean remove from the car then yes. The only part I actually did anything to was the rear U joint which I replaced. I then put the drive line back in.

Since I've been fighting with the brakes I've not yet had time to try rotating the driveline 180 degress and reattaching it but hopefully I will by tomorrow and give that a test run.

Come to think of it, I degreased and cleaned the shaft before I reattached it. Perhaps the lack of grime has thrown it off balance
 
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